Communication technology has progressed significantly in the past few years. Today, much information is carried over optical communications fiber. This technology, known as fiber optic technology allows the transport of information at data rates currently exceeding billions (10.sup.9) of bits of information per second. Part of the technology that enables this optical communication is the ability to direct light onto an optical fiber and switch that light appropriately. Typically, a number of optical fibers are combined into a fiber optic cable. When a fiber optic cable is carrying many individual signals over large distances, it is necessary to have the ability to switch those signals onto other fiber optic cables. A mesh of fiber optic cable infrastructure spans the world. At certain places in the mesh it is desirable to have the ability to switch the light signals from one fiber optic cable to another. A typical fiber optic cable may be comprised of a plurality of individual optical fibers bound together, for example, in a ribbon arrangement. A typical fiber optic ribbon cable may contain 32 individual optical fibers. Each optical fiber is capable of carrying one signal, or in the case of dense wave division multiplexing (DWDM), in which many signals may be multiplexed onto a single optical fiber through the use of multiple colors of light, each optical fiber may carry a plurality of light colors (wavelengths), each color carrying a single signal.
Optical switches capable of routing light from one direction to another have been known for some time. A new type of optical switch element is disclosed in commonly assigned U.S. Pat. No. 5,699,462 to Fouquet et al., in which an optical switch element is located at an intersection of two optical waveguides. Depending on the state of a material within the optical switch element, light is either transmitted through the switch element continuing axially on the original waveguide, or reflected by the switch element onto a waveguide that intersects the original waveguide. The switch element is filled with a material that, while in a transmissive state, has an index of refraction substantially equal to that of the waveguide, thus allowing light in the waveguide to pass through the switch element. The state of the material within the switch element may be changed, through the operation of heaters or the like within the switch element, so as to cause a gas, or bubble, to be formed within the switch element. While present in the switch element the bubble causes a refractive index mismatch between the waveguide and the switch element, thus causing the light in the waveguide to be reflected onto the intersecting waveguide. This state is known as the reflective state. The operation of a preferred and many alternative embodiments of this switch element is set forth in detail in commonly assigned U.S. Pat. No. 5,699,462 to Fouquet et al., which is hereby incorporated by reference.
When placed at an intersection of two waveguide segments, one of the above-mentioned optical switch elements forms an optical switch point, which may be used to switch signals on a plurality of optical fibers. The optical switch points may be further arranged so as to form a switching matrix. For example, when arranged in a 32.times.32 matrix, formed by 32 rows and 32 columns of optical switch points, a 32 fiber optic ribbon cable can be connected to 32 input lines and another 32 fiber optic ribbon cable can be connected to 32 output lines, the output lines intersecting the 32 input lines. Because a switch element is located at each optical switch point it is possible to switch any of the 32 input lines to any of the 32 output lines. In this manner, optical signals may be directed from one fiber optic cable onto another, resulting in a compact optical switch.
One drawback with the above-described optical switch is that it is not normally possible to test each of the switch elements while the switch is in operation.
Furthermore, another drawback with the above-described optical switch, is that, due to process variations during manufacture of switch elements and pressure changes during operation, it is necessary to calibrate during set up and to monitor during operation the heater voltage and current for each switch element such that a bubble of optimum size may be formed and maintained. Too small a bubble and the light may not be properly switched and too large a bubble and the bubble may escape, or the heater may fail. To perform this calibration with this matrix switch arrangement it would be necessary to have N calibrated light sources and M calibrated light receivers. As the dimensions of the switch approach N=32 and M=32, this arrangement would become prohibitively expensive with which to perform calibration and monitoring in use.
Therefore, it would be desirable to have a method by which to test, calibrate and to monitor the performance of an optical switching matrix.